[gas][arm] Add -mwarn-restrict-it

[gas][arm] Add -mwarn-restrict-it

Add a -m{no-}warn-restrict-it option to control IT related warnings in
ARMv8-A and ARMv8-R.  This is disabled by default.

Committed on behalf of Wilco Dijkstra.

gas/ChangeLog:
2019-12-11  Wilco Dijkstra  <wdijkstr@arm.com>

	* config/tc-arm.c (warn_on_restrict_it): Add new variable.
	(it_fsm_post_encode): Check warn_on_restrict_it.
	(arm_option_table): Add -mwarn-restrict-it/-mno-warn-restrict-it.
	* testsuite/gas/arm/armv8-2-fp16-scalar-bad.d: Add -mwarn-restrict-it.
	* testsuite/gas/arm/armv8-2-fp16-scalar-bad-ext.d: Likewise.
	* testsuite/gas/arm/armv8-a-bad.d: Likewise.
	* testsuite/gas/arm/armv8-a-it-bad.d: Likewise.
	* testsuite/gas/arm/armv8-r-bad.d: Likewise.
	* testsuite/gas/arm/armv8-r-it-bad.d: Likewise.
	* testsuite/gas/arm/sp-pc-validations-bad-t-v8a.d: Likewise.
	* testsuite/gas/arm/udf.d: Likewise.